Use PostgreSQL's built-in uuid
data type, and create a regular b-tree index on it.
There is no need to do anything special. This will result in an optimal index, and will also store the uuid
field in as compact a form as is currently practical.
(Hash indexes in PostgreSQL prior to version 10 were not crash-safe and were really a historical relic that tended to perform no better than a b-tree anyway. Avoid them. On PostgreSQL 10 they've been made crash-safe and had some performance improvements made so you may wish to consider them.)
If for some reason you could not use the uuid
type, you would generally create a b-tree on the text representation or, preferably, a bytea
representation of the uuid.
This is to implement the feature found in the standard. (copied from a draft, date: 2011-12-21):
4.15.11 Identity columns
The columns of a base table BT can optionally include not more than one identity column. The declared type
of an identity column is either an exact numeric type with scale 0 (zero), INTEGER
for example, or a distinct
type whose source type is an exact numeric type with scale 0 (zero). An identity column has a start value, an
increment, a maximum value, a minimum value, and a cycle option.
...
... The definition of an identity column
may specify GENERATED ALWAYS
or GENERATED BY DEFAULT
.
It is a property of a column which basically says that the values for the column will be provided by the DBMS and not by the user and in some specific manner and restrictions (increasing, decreasing, having max/min values, cycling if the max/min value is reached).
Sequence generators (usually called just "sequences") are a related SQL standard feature: it's a mechanism that provides such values - and can be used for identity columns.
Note the subtle difference: a SEQUENCE
is an object that can be used to provide values for one or more identity columns or even at will.
The various DBMS have so far implemented similar features in different ways and syntax (MySQL: AUTO_INCREMENT
, SQL Server: IDENTITY (seed, increment)
, PostgreSQL: serial
using SEQUENCE
, Oracle: using triggers, etc) and only recently added sequence generators (SQL Server in version 2012 and Oracle in 12c).
Up to now Postgres has implemented sequence generators (which can be used to provide values for column, either with the special macros serial
and bigserial
or with nextval()
function) but has not yet implemented the syntax for identity columns, as it is in the standard.
Defining identity columns (and the slight difference from serial
columns) and various syntax (eg. GENERATED ALWAYS
, NEXT VALUE FOR
, etc) from the SQL standard is what this feature is about. Some changes / improvements may need to be done on the implementation of sequences as well, as identity columns will be using sequences.
If you follow the link identitity columns (from the page you saw), you'll find:
identity columns
From: Peter Eisentraut
To: pgsql-hackers
Subject: identity columns
Date: 2016-08-31 04:00:42
Message-ID: 6adbacbf-73bc-dd1a-2033-63409180fd18@2ndquadrant.com
Here is another attempt to implement identity columns. This is a
standard-conforming variant of PostgreSQL's serial columns. It also
fixes a few usability issues that serial columns have:
- need to set permissions on sequence in addition to table (*)
CREATE TABLE
/ LIKE
copies default but refers to same sequence
- cannot add/drop serialness with
ALTER TABLE
- dropping default does not drop sequence
- slight weirdnesses because serial is some kind of special macro
(*) Not actually implemented yet, because I wanted to make use of the
NEXT VALUE FOR
stuff I had previously posted, but I have more work to
do there.
...
Update 2017, September: seems like the feature will be in Postgres 10, which is to be released in a few days/weeks: What's New In Postgres 10: Identity Columns
Oracle have also implemented identity columns and sequences, in version 12c. The syntax is according to the standard, as far as I checked:
Identity Columns in Oracle Database 12c Release 1 (12.1)
The 12c database introduces the ability define an identity clause against a table column defined using a numeric type. The syntax is show below.
GENERATED
[ ALWAYS | BY DEFAULT [ ON NULL ] ]
AS IDENTITY [ ( identity_options ) ]
Best Answer
It seems that currently PostgreSQL only provides the following
The PostgreSQL docs go on to say
So it seems it's sequence specific. The SLQ 2011 spec implies there is nothing more to it,